Category Archives: Grade 12

Diffusion in agar cubes

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Conclusion Questions:

In terms of maximizing diffusion, what was the most effective size cube that you tested? 

Why was that size most effective at maximizing diffusion?  What are the important factors that affect how materials diffuse into cells or tissues? 

  • Smaller surface area and total volume


If a large surface area is helpful to cells, why do cells not grow to be very large? 

  • A larger surface area may be helpful to a ce ll, however, diffusion and other processes take much longer to occur with an increase in cell size. Overall a larger cell increases the time some of these processes take which have an effect on the functions. Smaller cells are more effective with diffusion.

You have three cubes, A, B, and C.  They have surface to volume ratios of 3:1, 5:2, and 4:1 respectively.  Which of these cubes is going to be the most effective at maximizing diffusion, how do you know this? 

  • The cube with the ratio 5:2 will be the most effective at maximizing diffusion. (Reduced 5:2 = 2.5:1 which is the smallest). Smaller volume is more effective at diffusion.  

How does your body adapt surface area-to-volume ratios to help exchange gases? 

  • The cell membrane has a high enough surface area to serve the internal contents of the cell, including the exchange of gases. As the cell grows it’s surface area to volume ratio decreases, reducing the rate of gas exchange  

Why can’t certain cells, like bacteria, get to be the size of a small fish? 

  • They need to keep a good volume to surface area ratio. They do this by dividing and this keeps them small. Bacteria also depend on organelles rather than specialized cells.  

What are the advantages of large organisms being multicellular? 

  • Large organisms are multicellular because it allows them to have specialized cells for all of the required bodily functions. They can have longer life spans as they can continue living when individual cells die. Also allows for differentiation of cell types within one organism 

DNA Structure and Replication

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  1. Explain the structure of DNA – use the terms nucleotide, anti-parallel strands and complimentary base pairing

 

DNA is made up of molecules called nucleotides that contain a phosphate group, a sugar group, and a nitrogen base. There are 4 nitrogen bases that are linked with hydrogen bonds to their complementary base pairs, Adenine – Thymine and Cytosine – Guanine. DNA has 2 strands that are antiparallel, so the nucleotides are oriented in opposite direction. The 2 chains twist around each other creating the double helix.

 

  1. How does this activity help model the structure of DNA? What changes could we make to improve the accuracy of this model? Be detailed and constructive

 

This activity helped model the structure of DNA by showing each individual part of the structure and how everything is attached and bonded. I was able to see the general shape of DNA as well as have a better understanding of the how the backbone and nitrogen bases are paired together. Changes that could be made are using a different material as hydrogen bonds since it was difficult to make them stay onto the other pipe cleaners. It was also hard to twist the model into the double helix without it breaking. If we were to use a something stronger to hold the model together, I think it would’ve been easier to move around.

 

 

 

 

  1. When does DNA replication occur?

 

Occurs prior to cell division. DNA replication is semi-conservative, it uses its original backbone to replicate DNA.

 

  1. Name and describe the 3 steps involved in DNA replication. Why does the process occur differently on the “leading” and “lagging” strands?

 

Unwinding & unzipping – helicase

Complimentary base pairing – polymerase

Joining of adjacent nucleotides – ligase

 

In the leading strand the parent strand is read by the DNA polymerase from 3’ to 5’.

In the lagging strand the parent strand is read by the DNA polymerase from 5’ to 3’, the polymerase can only read from the 3’ direction and the complimentary bases are backwards. The complimentary strand is built by DNA polymerase in short segments moving backwards.

 

  1. The model today wasn’t a great fit for the process we were exploring. What did you do to model the complimentary base pairing and joining of adjacent nucleotides steps of DNA replication? In what ways was this activity well suited to showing this process? In what ways was it inaccurate.

 

We took pictures of the green playdoh splitting the DNA into 2 strands and then used the orange playdoh to represent the polymerase and complimentary base pairing by putting the playdoh between the white pipe cleaners to show that it wasn’t connected yet but was being built. We then used the blue playdoh to represent ligase by placing the playdoh on top on our white pipe cleaners to show that the adjacent nucleotides were joined together. This activity was a good way to show how the steps work but it was hard to show it in the pictures. In was inaccurate in showing that the lagging stand works backwards, and the strand is built in small segments. Since we had to place the beads on before the pipe cleaners, we weren’t actually building the lagging strand in segments we were building it the same way the leading strand was built just in the opposite direction.

 

Helicase                                    Polymerase                              Ligase